The invention relates to a steering system for heavy vehicles in which tight cornering and a high degree of maneuverability are important features. In particular, the invention relates to a steering system in which the front and rear wheels of a vehicle can be operated in a variety of modes, including front only, coordinated (or radiarc), crab, and rear/front independent.
Systems for controlling the steering of vehicles have included mechanical systems like the one shown in U.S. Pat. No. 3,903,983, in which cables and related mechanisms are arranged so that a rear steering lever is made to mechanically follow or depend upon front wheel steering. U.S. Pat. No. 4,175,638 shows an electronically controlled four wheel steering system in which electronic controls for selecting steering modes and for independently controlling rear wheel steering are mounted adjacent to one another on a lever or control panel.
The steering systems discussed above, and other prior art steering systems generally, have lacked several important features relating to safety, reliability and versatility. Clearly displaying the status of various aspects of the system has been limited. Complete and exact status information is important for safe operation. Examples of vehicles where four wheel steering is used, both on and off road, are runway sweeping machines, tow-tractors, refueling vehicles, crash/fire rescue vehicles, snow-plowing and blower vehicles, and military on-off road all-wheel drive vehicles.
It is an object of the present invention to provide a steering system which is efficient and which provides the operator with a variety of important system status information.
Another object of the invention is to provide a steering system which can be safely operated in a variety of modes.
Another object of the invention is to provide a steering system which limits the speed at which the vehicle can be operated while in a particular mode.
Another object is to provide a steering system in which various controls can be adjusted to meet the needs of a particular user or a particular application.
A further object is to provide a steering system in which modes may be changed while the vehicle is in motion.
Yet another object of the present invention is to provide a steering system in which the angle of disposition of the rear wheels is graphically displayed in order to assist the operator.
These and other objects and advantages of the invention are obtained with a steering system which has front end and rear sensors which generate a signal corresponding to the position of the wheels of a vehicle. The sensors are part of a circuit which is used to control the steering system. The control unit can be set to a variety of steering modes so that the vehicle can be operated in at least four different ways, namely, "front only", "coordinated" (rear wheel steering movement opposite to that of the front), "crab" (rear and front move the same way), and "independent" (rear and front wheels are independently controlled). The system includes a rear steering lockout which is comprised of a pin which prevents steering of the rear wheels. The lockout may be set to activate at a predetermined vehicle speed. The system also includes a way of adjusting the range of steering angles through which the wheels are allowed to move.